Description: Thallus crustose, fully endosubstratic, usually covering large surfaces, grey, dark grey-green or blue-green, sometimes dark lead-grey, often slightly glossy, uncracked. Cortex (lithocortex) densely prosoplectenchymatous, of conglutinated hyphae intermingled with microcrystals, with a green pigment; algal layer discontinuous, photobiont cells in clusters; medulla prosoplectenchymatous, sometimes with oil cells (macrospheroids) in the lower part. Perithecia black, completely immersed in the rock and leaving deep pits when they fall off. Involucrellum black, flat to slightly convex, much narrower than the body of the perithecium (0.15-0.22 μm diam.), spreading sideways but scarcely downwards, in surface view usually with 3-6 fine cracks radiating from ostiole; exciple 0.2-0.3 mm across, the wall colourless to pale brown; hymenial gel hemiamyloid, I+ red (I+ blue at very low concentrations of I), K/I+ blue; hamathecium of periphyses in the ostiolar canal, and short periphysoids in the upper half of the perithecial cavity; paraphyses dissolving early. Asci 8-spored, two-layered, fissitunicate, with a thin tholus reacting I-, Verrucaria-type. Ascospores 1-celled, hyaline, ellipsoid, often poorly developed, 14-21(-25) x 6-8(-12) μm. Photobiont chlorococcoid. Spot tests: K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a mild-temperate lichen found on compact limestone and dolomite in sheltered situations (e.g. in forests), with optimum in submediterranean areas; in the Mediterranean belt it is confined to humid-shaded situations, reaching near treeline in the mountains of the South; forms from southern Italy with a dark grey thallus are common, and deserve further study.